the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Aug 2025
Internal tides on the Al-Batinah shelf: evolution, structure and predictability
Abstract. Internal tides are a key mechanism of energy transfer on continental shelves. We present observations of internal tides on the northern Oman shelf based on moored temperature and velocity records collected during summer 2022. The regional shelf exhibits strong summer stratification, supporting shoreward-propagating internal tides with pronounced fortnightly modulation in amplitude and energy fluxes. Despite semidiurnal dominance in barotropic forcing, the internal tides appear predominantly in the diurnal band. Waveform structures undergo transition from quasi-linear depression waves to increasingly nonlinear features, including steepening, asymmetry, and polarity reversal. Modal decomposition shows a shift toward first-mode dominance as the thermocline deepens seasonally. Cross-shelf coherence and phase-speed estimates confirm that the observed internal tides maintain spatial coherence from the shelf edge to the shallow inner shelf beyond the typical internal surf zone. Predictability skill scores indicate that the local internal tides are comparable to high-predictability sites globally while inshore directed energy flux, diurnal dominance and phase lags to barotropic forcing still indicate remote generation.
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Status: open (until 24 Oct 2025)
- RC1: 'Comment on egusphere-2025-4158', Johannes Becherer, 02 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-4158', Anonymous Referee #2, 16 Oct 2025
reply
Review of ”Internal tides on the Al-Batinah shelf: evolution, structure and predictability” by Bruss et al.
This paper presents a detailed analysis of measurements on a shelf in the Gulf of Oman that show predominantly diurnal internal tides. The results are carefully described and offer a valuable contribution to the understanding of internal tides in that region.
I recommend publication after a minor revision, in which the following points are considered.
The authors suggest that the diurnal internal tides may be generated remotely because locally the barotropic tidal current is mainly semidiurnal. This could be correct, but it would be worthwhile to consider the local setting more fully, perhaps with a simple internal-tide generation model. Diurnal and semidiurnal internal tidal beams have very different slopes and the local bathymetry may be more favorable to generate one or the other.
Another argument put forward in the paper is the shift and variability in fortnightly cycles between barotropic and baroclinic signals. However, such shifts have been demonstrated to occur even in locally generated internal tides, because the M2 and S2 (or similarly K1 and O1) beams propagate at different angles, creating spatially varying phase shifts in the baroclinic fortnightly cycles, plus a sensitivity to time-varying background stratification.
As a general point, I think the paper could be shortened (leaving out details that do not really add much to the story), highlighting the main findings. It is now quite a long paper on a relatively limited dataset.
Besides, I notice a number of smaller points that deserve attention in a revision:
1) line 88, if rho denotes (in-situ) density then the expression for N2 should have a term involving the speed of sound as well.
2) eq. (7) is based on the hydrostatic approximation (which is fine for internal tides), not “non-hydrostatic” as stated in line 167.
3) In figure 3d, what direction do the vectors refer to? (cross-slope?)
4) In line 422, the authors state, as if it were evident, that internal tidal frequencies can deviate by “Doppler shifting”. How? In any situation where both the source and observer are fixed in space (as is the situation here), no Doppler shifts occur (and the presence of a mean flow does not change this!). In other words, this statement needs a more careful consideration (or perhaps removal).
Citation: https://doi.org/10.5194/egusphere-2025-4158-RC2
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- 1
The manuscript presents data from two moorings on the Al-Batinah shelf and provides a detailed analysis of the internal tide at this site, covering most commonly used characteristics. This work is a valuable contribution to the literature on continental shelf internal tides, adding a new data point to the global record. Notable findings include a predominantly diurnal internal tide despite stronger semidiurnal forcing, a subharmonic peak at 1/2 M2, and a high predictability score despite likely remote generation. The manuscript is well written, and the figures are of excellent quality. I recommend publication after minor revisions.
Specific comments:
L 32: (~20km) is not really wide compared to other shelf regions. I would still call that a rather narrow shelf compared for instance to the US west coast or the NW European shelf.
L48: steep -> sharp?
L65: I don't get it. What is the 285-115° axis?
L83: 15? resolutions
L90f: I am confused by this zTC calculations. You say you calculate it not along a fixed isotherm but as a fixed DT to SST. SST comes from satellite, right? So how consistent is that. I was also curious how a diurnal change in SST would affect zTC, but I guess the satellite data is daily averaged? If the diurnal SST signal is included this could introduce an artificial diurnal signal in zTC. Please clarify.
Also, Is there a diurnal sea breeze that could influence the diurnal signal?
L91 ztc -> zTC
L106: So if your diurnal band includes the inertial frequency, how can you be sure that your diurnal tide calculations are not contaminated by inertial motions?
eq5: You don't introduce Psi
L255: I cannot see stratification breaking down in Fig.3b
L259: Where does this S2 estimate come from. It looks like internal wave shear can be much larger at times. Are you sure that Ri > 1/4 always? This seems a bit handwavy.
L264 How can you be so sure they have not yet reached saturation?
Fig 3:
- why is PEA jumping around so much?
- panel c: I don't understand the triangles. In general I feel that such a complex figure requires a more detailed caption.
- panel d: I don't understand what the background lines represent? Are they described in the caption.
- The arrows indicating energy flux are nice, but they make it hard to gauge the magnitude. I wonder if you should add a panel with the fluxes magnitude as a time series.
L281: I wonder if the delay in the fortnightly cycle could help you with the remote generation argument. If you can pinpoint the delay and combine it with the phase speed of the internal tide you might be able to estimate a distance to the generation site.
L285: This statement is not quite right. While the flux we observed in California dropped by at least an order of magnitude at 25m, it is still at a comparable level to the fluxes you observe O(1-3) W/m. This meant full saturation in our case, because our stratification was weaker than yours.
On that note: Since you think that you are still outside of the saturation regime, it would be interesting to estimate where the saturation regime would start in your case. You could use (eq 15, Becherer et. al.) to calculate this. From my rough back of the envelope calculation I get around 12-15m depth, which is not too far away from your shallow mooring.
You could also check if you are really outside of the saturation regime by estimating the saturated flux based on your observed top to bottom density difference (eq 10, Becherer et. al.) and compare it to your observed fluxes.
Another way to check would be to compare the IT signal observed at RAS with ISQ and see if you can detect already some amplitude decrease. You sort of do this in Fig5a, where it looks like that could be a decay at least in the early part of the record. So I am not 100% convinced yet that you are outside of the saturation regime.
L296: What do you mean by IT intensity here? This causal connection between IT intensity (??) and mixing implies that mixing is happening close to your site, which also implies that the IT is already substantially dissipating, which in turn implies that it reached saturation?
L299: But how far onshore? See comment above for saturation regime.
L300: You should also cite McSweeney et al. 2020 here (reference below). This reference could also be interesting in the context of the phase speed estimates and the polarity reversal.
L303: What is your temporal resolution. Are you saying that you cannot resolve the buoyancy period?
Fig4: - You need a colormap here. Otherwise this figure cannot be used as stand alone.
L350: "were derived from zero-crossing N²(z) profiles" How can the N2 profile have zero crossings? Do you mean density profile here?
L381: I don't follow this argument. How does the coherence confirms the DWL do not influence IT?
L406: This is again a good place for the McSweeney reference.
L412: The advection by background currents should be testable with your ADCP data.
L423: This suggests that the distance of the two moorings is smaller than the wavelength, which probably also explains the large coherence.
Section 3.4: It is not clear why you compare the NW position here. Do you have good reasons to think that this could be the generation site for the observed ITs? If yes please explain. You could also test if the distance matches the delay in the fortnightly cycle you observe, which would be a nice consistency check.
Fig6:
Fig7:
Ref:
McSweeney, J. M., and Coauthors, 2020: Observations of Shoaling Nonlinear Internal Bores across the Central California Inner Shelf. J. Phys. Oceanogr., 50, 111–132, https://doi.org/10.1175/JPO-D-19-0125.1.